Put it on the market and you might be able to swap a townhouse in Rockville for a townhouse in, say, Woodbridge. But list the same place in one of the vacation-exchange club catalogues and you could end up swapping it for a secluded Jamaican mini-villa - at least for a couple of weeks.

That's what happened the first time Virginia Grimminger registered with the New York-based Vacation Exchange Club. The Jamaican couple who contacted her had urgent business in Washington and didn't have time to shop around. So Grimminger, a retired government worker, traded her three-bedroom condominium for their Jamaican estate.

"I can't begin to tell you how much I saved," she says. "It was a gorgeous home that had been in the family since colonial days. The property itself was exquisite - we had our own swimming pool and a private snorkeling cove. And there was a household staff that came with the deal - a gardener, houseman, cook and laundress. It was a real luxury."

That experience was all it took to convince Grimminger of the merits of vacation exchanging - an increasingly popular plan in which two families, rather than staying in hotels, simply spend their vacations living in each other's houses.

But house-swapping, enthusiasts say, is more than a way to bypass the tourist annoyances of boring hotel rooms and monotonous restaurant meals. When you trade houses with someone, you get to know their country. You drive their car, shop in their grocery stores, meet their neighbors.

You also save a lot of money - not the least of the reasons for swapping's growing appeal. "Within the last three years the inflationary problem has created a lot of interest in exchanging," says Mary DeBaldo, manager of New York's Vacation Exchange Club. The largest of the U.S. exchange organizations, it started out in 1960 with about a hundred subscribers and now lists more than 5,000.

The opportunities for savings are obvious. When you trade, your only vacation expenses are transportation and food. Lodging, which can be a very expensive item in traveling around is free. And with the use of a car, which many exchangers throw in, you can take day trips and use the home as a base - an inexpensive way to see a lot more of the country.

It's also no small comfort to know your own house is being lived in, cats fed, plants watered, grass mowed and so forth. You can arrange any kind of deal you want.

But it's the personal experiences, DeBaldo says, that keep their clients coming back for more each year. "You can't measure what you've learned culturally on an exchange," she says. "Usually people have some experience that's so different you can't pay for it. They're so grateful."

Like the Chevy Chase family who spent the summer in Ireland. James and Kathy Gray, both university professors, used one exchange club to rent out their house here (to a Vermont man who was working in Washington for the summer) and another to find a comparable house to rent in Dublin. "It's a great way to travel with little kids," Kathy Gray says."They made friends with the kids on the block, we got to know the neighbors. We took overnight trips and really got to know Ireland. It was a marvelous experience.

Or Larry and Anne Pearl, a Washington couple who discovered that swapping also works on a smaller scale. "We wrote and asked some people who lived in New York if they'd like to exchange just for a weekend," Anne Pearl recalls. The New York couple said yes - and instead of staying in a hotel room, the Pearls spent the weekend in a loft in the city.

Or the German professor who spent some time in Florida. He got to know the people in the neighborhood so well that they had what they called a Peterfest before he left. "He comes back now and he's treated as a neighbor, not a visitor," says DeBaldo. "Those are experience money can't buy."

So why on earth would anyone want to stay in a hotel? People in the business are the first to admit that house-swapping is not for everybody. The exchange clubs take no responsibility for anything that may go wrong, and some people realize that more acutely than others. They're perpetual nervous wrecks worrrying about waht migh happen to their home in the hands of strangers.

But theoretically, each swapper knows the other has his house at his mercy. This gives each some leverage.

That reasoning seems to be enough to satisfy most house-swappers. "We had no worries whatsoever," says Kathy Gray. "The neighbors thought I was crazy, but all I did was put away the good china and glassware. When we came back, the house was absolutely in immaculate shape."

Living in Washington can be a definite advantage in attracting a potential exchanger. As Virginia Grimminger found, people may be willing to "swap down" for the convenience of the location. But successful swappers say a lot depends on your presentation, and on how aggressive you are. Grimminger sent out 75 query letters the first time she swapped, and got about a dozen responses - including the one from the Jamaican villa owners.

There are also some basic common-sense things to do, the experts say, that will minimize potential problems. Ask for - and check - the references of the people you swap with. Make sure your insurance coverage is adequate. If you're nervous about your valuables, don't swap with families who have small children. or lock up your fancy knickknacks while you're gone.

For the truly discerning, there are organizations like Inquiline, which bills itself as a service designed for executives and professionals. Ben Kernan, the president, says they urge subscribers to supply as much information about themselves as they do about their homes. Their directory sounds like a little "Who's Who," with information on subscribers' occupations, education, associations and so forth.

Most clubs just stick to listing people's occupations, though, and a quick flick through any of the directories shows that most swappers are professionals. It gets to be monotonous after a while: lawyers, professors, economists, federal government workers . . .

Virginia Grimminger checks out her potential swappers just by corresponding with them. When she was planning her first exchange, "There were quite a few letters that went back and forth between us. We talked about everything and exchanged photos. That way, you get a feeling for the family."

It's an attitude common among house swappers.

"I personally don't have a lot of trepidation about thinking anyone's going to clean me out," says Sandra Jenkins, a Washington attorney who's negotiating her first exchange. "It's not like people who blow into a motel and are never heard from again.

"Besides, there's very little that can't be replaced by my insurance policy. Household possessions, when you get right down to it, are not the equivalent of a human experience." SWAP SHOPS

In most cases, people send in a brief description of their house (photo optional) and location and pay a fee to be listed in an exchange directory, usually for a year. That's the extent of the exchange services' responsibility - unless otherwise specified, you're expected to work out the details yourself. Most directories come out in the early spring, so that those registering now would be too late to be included in the 1978 editions. But you can subscribe to the directory without being listed in it yourself. Sometimes it's cheaper to do it this way.

I'ts worth repeating that none of the organizations listed below takes any responsibility for anything that might go wrong.

VACATION EXCHANGE CLUB

350 Broedway

New York, N.Y. 10013

212/966-2576

The largest of the U.S. exchange clubs, with about 5,000 listings and affiliates in England, Australia, Israel and Hawaii. Deadline for inclusion in their 1978 directory was Feb. 15, but copies are available through September and you can subscribe without being listed yourself. Cost is $10 without a listing, $13 with. There's also a package deal: Buy this year's edition without being listed, and get next year's with a listing, for a total of $16.50.

INQUILINE

35 Adams St.

Bedford Hills, N.Y. 10507

914/241-0102

Inquiline lists about 600 homes in its directory, but has 2,000 subscribers, perhaps because it costs so much more to be listed: $30 to subscribe, $30 to list or $50 to do both. For $100 more, they'll do the matching. No extra charge for photos. Two directories (the 1979 edition comes out in November) and four newsletters with travel information, guidelines and subscribers' experiences. Rentals and host programs also available.

INTERCHANGE HOME EXCHANGE

Main office: 26 Omaha Ave.

Toronto M5J 1Z8 Canada

416/366-4887

Branch office: 888 7th Ave., Suite 400

New York, N.Y. 10019

212/265-4600

About 500 listings. The annual directory ($7) doesn't publish names or address. Interested people contact Interchange, which then contacts both sides, checks references and arranges the swap. If both sides agree, each then pays a fee, usually about a hundred dollars. Rentals also available.

HOLIDAY HOME EXCHANGE, INC.

Box 555

Grants, N.M. 87020

505/287-3709

About 500 or 600 listings. The $15 fee covers a year's listing in a revolving directory (join anytime).

ADVENTURES LIVING

Box 278

Winnetka, III. 60093

312/446-9522

About 500 or 600 listings. The $25 fee covers a listing (the main edition comes out in January), two supplements and an instruction booklet.